In spite of legal objections, the European Union (EU) in May 2013 gave the conditional green light for the transfer of arms to the Syrian Opposition Council. The EU's decision is not a solitary move. Several other States, including Russia, the United States, Qatar and Saudi Arabia, have provided arms, funding and/or “non-lethal assistance” either to the Syrian government or to rebel forces combatting the Assad regime. The present contribution aims to shed light on the legality of such assistance. On the one hand, it assesses legal objections related to the fact that third-State assistance is used for the commission of widespread war crimes and human rights violations. On the other hand, it examines the compatibility of such assistance with the non-intervention principle and, in so doing, examines to what extent the latter principle discriminates between de jure governments and non-State armed groups in the context of a civil war.